Environmental Programming of Susceptibility and Resilience to Stress in Adulthood in Male Mice

Early-life stress induces persistent astrocyte dysfunction associated with fear generalisation

Early-life stress can have lifelong consequences, enhancing stress susceptibility and resulting in behavioural and cognitive deficits. While the effects of early-life stress on neuronal function have been well-described, we still know very little about the contribution of non-neuronal brain cells. Investigating the complex interactions between distinct brain cell types is critical to fully understand how cellular changes manifest as behavioural deficits following early-life stress. Here, using male and female mice we report that early-life stress induces anxiety-like behaviour and fear generalisation in an amygdala-dependent learning and memory task. These behavioural changes were associated with impaired synaptic plasticity, increased neural excitability, and astrocyte hypofunction. Genetic perturbation of amygdala astrocyte function by either reducing astrocyte calcium activity or reducing astrocyte network function was sufficient to replicate cellular, synaptic, and fear memory generalisation associated with early-life stress. Our data reveal a role of astrocytes in tuning emotionally salient memory and provide mechanistic links between early-life stress, astrocyte hypofunction, and behavioural deficits.

Early life stress and altered social behaviors: A perspective across species

Childhood and adolescent affiliations guide how individuals engage in social relationships throughout their lifetime and adverse experiences can promote biological alterations that facilitate behavioral maladaptation. Indeed, childhood victims of abuse are more likely to be diagnosed with conduct or mood disorders which are both characterized by altered social engagement. A key domain particularly deserving of attention is aggressive behavior, a hallmark of many disorders characterized by deficits in reward processing. Animal models have been integral in identifying both the short- and long-term consequences of stress exposure and suggest that whether it is disruption to parental care or social isolation, chronic exposure to early life stress increases corticosterone, changes the expression of neurotransmitters and neuromodulators, and facilitates structural alterations to the hypothalamus, hippocampus, and amygdala, influencing how these brain regions communicate with other reward-related substrates. Herein, we describe how adverse early life experiences influence social behavioral outcomes across a wide range of species and highlight the long-term biological mechanisms that are most relevant to maladaptive aggressive behavior.

Methods for Modeling Early Life Stress in Rodents

Animal models of early life stress/adversity (ELS) have provided a foundation from which our understanding of the psychoneuroimmunology of childhood trauma has expanded over recent decades. Rodent models are a cornerstone of the ELS literature with many studies utilizing paradigms based on early life separation/deprivation protocols and manipulating the cage environment. However, no animal model is perfect. In particular, the lack of standardization across ELS models has led to inconsistent results and raised questions regarding the translational value of common preclinical models. In this chapter, we present an overview of the history of ELS rodent models and discuss considerations relevant to the ongoing efforts to both improve existing models and generate novel paradigms to meet the evolving needs of molecular- and mechanism-based ELS research.

Stress resilience is an active and multifactorial process manifested by structural, functional, and molecular changes in synapses

Stress resilience is the ability of neuronal networks to maintain their function despite the stress exposure. Using a mouse model we investigate stress resilience phenomenon. To assess the resilient and anhedonic behavioral phenotypes developed after the induction of chronic unpredictable stress, we quantitatively characterized the structural and functional plasticity of excitatory synapses in the hippocampus using a combination of proteomic, electrophysiological, and imaging methods. Our results indicate that stress resilience is an active and multifactorial process manifested by structural, functional, and molecular changes in synapses. We reveal that chronic stress influences palmitoylation of synaptic proteins, whose profiles differ between resilient and anhedonic animals. The changes in palmitoylation are predominantly related with the glutamate receptor signaling thus affects synaptic transmission and associated structures of dendritic spines. We show that stress resilience is associated with structural compensatory plasticity of the postsynaptic parts of synapses in CA1 subregion of the hippocampus.

The interplay of serotonin 5-HT1A and 5-HT7 receptors in chronic stress

Serotonin regulates multiple physiological and pathological processes in the brain, including mood and cognition. The serotonin receptors 5-HT1AR (also known as HTR1A) and 5-HT7R (also known as HTR7) have emerged as key players in stress-related disorders, particularly depression. These receptors can form heterodimers, which influence their functions. Here, we explored the developmental dynamics of 5-HT1AR and 5-HT7R expression and validated heterodimerization levels in the brain of control and stressed mice. In control animals, we found that there was an increase in 5-HT1AR expression over 5-HT7R in the prefrontal cortex (PFC) and hippocampus during development. Using a chronic unpredictable stress as a depression model, we found an increase in 5-HT7R expression exclusively in the PFC of resilient animals, whereas no changes in 5-HT1AR expression between control and anhedonic mice were obtained. Quantitative in situ analysis of heterodimerization revealed the PFC as the region exhibiting the highest abundance of 5-HT1AR-5-HT7R heterodimers. More importantly, upon chronic stress, the amount of heterodimers was significantly reduced only in PFC of anhedonic mice, whereas it was not affected in resilient animals. These results suggest an important role of brain-region-specific 5-HT1AR-5-HT7R heterodimerization for establishing depressive-like behaviour and for development of resiliency.

Male and female mice respectively form stronger social aversive memories with same and different sex conspecifics

Mice offer a wealth of opportunities for investigating brain circuits regulating multiple behaviors, largely due to their genetic tractability. Social behaviors are of translational relevance, considering both mice and humans are highly social mammals, and disruptions in human social behavior are key symptoms of myriad neuropsychiatric disorders. Stresses related to social experiences are particularly influential in the severity and maintenance of neuropsychiatric disorders like anxiety disorders, and trauma and stressor-related disorders. Yet, induction and study of social stress in mice is disproportionately focused on males, influenced heavily by their natural territorial nature. Conspecific-elicited stress (i.e., defeat), while ethologically relevant, is quite variable and predominantly specific to males, making rigorous and sex-inclusive studies challenging. In pursuit of a controllable, consistent, high throughput, and sex-inclusive paradigm for eliciting social stress, we have discovered intriguing sex-specific social aversions that are dependent upon the sex of both experimental and conspecific mice. Specifically, we trained male and female F1 129S1/SvlmJ × C57BL/6J mice to associate (via classical conditioning) same or different sex C57BL/6J conspecifics with a mild, aversive stimulus. Upon subsequent testing for social interaction 24 h later, we found that males socially conditioned better to male conspecifics by exhibiting reduced social interaction, whereas females socially conditioned better to male conspecifics. Serum corticosterone levels inversely corresponded to social avoidance after different sex, but not same sex, conditioning, suggesting corticosterone-mediated arousal could influence cross sex interactions. While our paradigm has further optimization ahead, these current findings reveal why past pursuits to develop same sex female social stress paradigms may have met with limited success. Future research should expand investigation of utilizing male mouse conspecifics to instigate social stress across sexes.

Activity-Dependent Synaptic Plasticity in the Medial Prefrontal Cortex of Male Rats Underlies Resilience-Related Behaviors to Social Adversity

Individuals considered resilient can overcome adversity, achieving normal physical and psychological development, while those deemed vulnerable may not. Adversity promotes structural and functional alterations in the medial prefrontal cortex (mPFC) and hippocampus. Moreover, activity-dependent synaptic plasticity is intricately linked to neuronal shaping resulting from experiences. We hypothesize that this plasticity plays a crucial role in resilience processes. However, there is a notable absence of studies investigating this plasticity and behavioral changes following social adversity at different life stages. Consequently, we evaluated the impact of social adversity during early postnatal development (maternal separation [MS]), adulthood (social defeat [SD]), and a combined exposure (MS + SD) on behavioral outcomes (anxiety, motivation, anhedonia, and social interaction). We also examined cFos expression induced by social interaction in mPFC and hippocampus of adult male rats. Behavioral analyses revealed that SD-induced anhedonia, whereas MS + SD increased social interaction and mitigated SD-induced anhedonia. cFos evaluation showed that social interaction heightened plasticity in the prelimbic (PrL) and infralimbic (IL) cortices, dentate gyrus (DG), CA3, and CA1. Social interaction-associated plasticity was compromised in IL and PrL cortices of the MS and SD groups. Interestingly, social interaction-induced plasticity was restored in the MS + SD group. Furthermore, plasticity was impaired in DG by all social stressors, and in CA3 was impaired by SD. Our findings suggest in male rats (i) two adverse social experiences during development foster resilience; (ii) activity-dependent plasticity in the mPFC is a foundation for resilience to social adversity; (iii) plasticity in DG is highly susceptible to social adversity.

The impact of pubertal stress and adult hormone exposure on the transcriptome of the developing hypothalamus

Why individuals suffer negative consequences following stress is a complex phenomenon that is dictated by individual factors, the timing of stress within the lifespan, and when in the lifespan the consequences are measured. Women who undergo adverse childhood experiences are at risk for lasting biological consequences, including affective and stress dysregulation. We have shown that pubertal adversity is associated with a blunted hypothalamic-pituitary-adrenal axis glucocorticoid response in peripartum humans and mice. In mice, our prior examination of the paraventricular nucleus (PVN) of the hypothalamus showed that pubertal stress led to an upregulation of baseline mRNA expression of six immediate early genes (IEGs) in the PVN of adult, pregnant mice. Separately, we showed that the pregnancy-associated hormone allopregnanolone is necessary and sufficient to produce the blunted stress response phenotype in pubertally stressed mice. In the current study, we further examined a potential mechanistic role for the IEGs in the PVN. We found that in pubertally stressed adult female, but not male, mice, intra-PVN allopregnanolone was sufficient to recapitulate the baseline IEG mRNA expression profile previously observed in pubertally stressed, pregnant mice. We also examined baseline IEG mRNA expression during adolescence, where we found that IEGs have developmental trajectories that showed sex-specific disruption by pubertal stress. Altogether, these data establish that IEGs may act as a key molecular switch involved in increased vulnerability to negative outcomes in adult, pubertally stressed animals. How the factors that produce vulnerability combine throughout the lifespan is key to our understanding of the etiology of stress-related disorders.

Adolescent environmental enrichment induces social resilience and alters neural gene expression in a selectively bred rodent model with anxious phenotype

Stress is a major influence on mental health status; the ways that individuals respond to or copes with stressors determine whether they are negatively affected in the future. Stress responses are established by an interplay between genetics, environment, and life experiences. Psychosocial stress is particularly impactful during adolescence, a critical period for the development of mood disorders. In this study we compared two established, selectively-bred Sprague Dawley rat lines, the "internalizing" bred Low Responder (bLR) line versus the "externalizing" bred High Responder (bHR) line, to investigate how genetic temperament and adolescent environment impact future responses to social interactions and psychosocial stress, and how these determinants of stress response interact. Male bLR and bHR rats were exposed to social and environmental enrichment in adolescence prior to experiencing social defeat and were then assessed for social interaction and anxiety-like behavior. Adolescent enrichment caused rats to display more social interaction, as well as nominally less social avoidance, less submission during defeat, and resilience to the effects of social stress on corticosterone, in a manner that seemed more notable in bLRs. For bHRs, enrichment also caused greater aggression during a neutral social encounter and nominally during defeat, and decreased anxiety-like behavior. To explore the neurobiology underlying the development of social resilience in the anxious phenotype bLRs, RNA-seq was conducted on the hippocampus and nucleus accumbens, two brain regions that mediate stress regulation and social behavior. Gene sets previously associated with stress, social behavior, aggression and exploratory activity were enriched with differential expression in both regions, with a particularly large effect on gene sets that regulate social behaviors. Our findings provide further evidence that adolescent enrichment can serve as an inoculating experience against future stressors. The ability to induce social resilience in a usually anxious line of animals by manipulating their environment has translational implications, as it underscores the feasibility of intervention strategies targeted at genetically vulnerable adolescent populations.

Early-Life Resource Scarcity in Mice Does Not Alter Adult Corticosterone or Preovulatory Luteinizing Hormone Surge Responses to Acute Psychosocial Stress

Early-life stressors can affect reproductive development and change responses to adult stress. We tested if resource scarcity in the form of limited bedding and nesting (LBN) from postnatal days (PND) 4 to 11 delayed sexual maturation in male and female mice and/or altered the response to an acute, layered, psychosocial stress (ALPS) in adulthood. Contrary to the hypotheses, age and mass at puberty were unaffected by the present application of LBN. Under basal conditions and after ALPS, corticosterone concentrations in males, diestrous females, and proestrous females reared in standard (STD) or LBN environments were similar. ALPS disrupts the luteinizing hormone (LH) surge in most mice when applied on the morning of proestrus; this effect was not changed by resource scarcity. In this study, the paucity of effects in the offspring may relate to a milder response of CBA dams to the paradigm. While LBN dams exited the nest more often and their offspring were smaller than STD-reared offspring on PND11, dam corticosterone concentrations were similar on PND11. To test if ALPS disrupts the LH surge by blunting the increase in excitatory GABAergic input to gonadotropin-releasing hormone (GnRH) neurons on the afternoon of proestrus, we conducted whole-cell voltage-clamp recordings. The frequency of GABAergic postsynaptic currents in GnRH neurons was not altered by LBN, ALPS, or their interaction. It remains possible that ALPS acts at afferents of GnRH neurons, changes response of GnRH neurons to input, and/or alters pituitary responsiveness to GnRH and that a more pronounced resource scarcity would affect the parameters studied.

Neurobiological basis of stress resilience

A majority of humans faced with severe stress maintain normal physiological and behavioral function, a process referred to as resilience. Such stress resilience has been modeled in laboratory animals and, over the past 15 years, has transformed our understanding of stress responses and how to approach the treatment of human stress disorders such as depression, post-traumatic stress disorder (PTSD), and anxiety disorders. Work in rodents has demonstrated that resilience to chronic stress is an active process that involves much more than simply avoiding the deleterious effects of the stress. Rather, resilience is mediated largely by the induction of adaptations that are associated uniquely with resilience. Such mechanisms of natural resilience in rodents are being characterized at the molecular, cellular, and circuit levels, with an increasing number being validated in human investigations. Such discoveries raise the novel possibility that treatments for human stress disorders, in addition to being geared toward reversing the damaging effects of stress, can also be based on inducing mechanisms of natural resilience in individuals who are inherently more susceptible. This review provides a progress report on this evolving field.

Adolescent environmental enrichment induces social resilience and alters neural gene expression in a selectively bred rodent model with anxious phenotype

Stress is a major influence on mental health status; the ways that individuals respond to or copes with stressors determine whether they are negatively affected in the future. Stress responses are established by an interplay between genetics, environment, and life experiences. Psychosocial stress is particularly impactful during adolescence, a critical period for the development of mood disorders. In this study we compared two established, selectively-bred Sprague Dawley rat lines, the "internalizing" bred Low Responder (bLR) line versus the "externalizing" bred High Responder (bHR) line, to investigate how genetic temperament and adolescent environment impact future responses to social interactions and psychosocial stress, and how these determinants of stress response interact. Male bLR and bHR rats were exposed to social and environmental enrichment in adolescence prior to experiencing social defeat and were then assessed for social interaction and anxiety-like behavior. Adolescent enrichment caused rats to display more social interaction, as well as nominally less social avoidance, less submission during defeat, and resilience to the effects of social stress on corticosterone, in a manner that seemed more notable in bLRs. For bHRs, enrichment also caused greater aggression during a neutral social encounter and nominally during defeat, and decreased anxiety-like behavior. To explore the neurobiology underlying the development of social resilience in the anxious phenotype bLRs, RNA-seq was conducted on the hippocampus and nucleus accumbens, two brain regions that mediate stress regulation and social behavior. Gene sets previously associated with stress, social behavior, aggression and exploratory activity were enriched with differential expression in both regions, with a particularly large effect on gene sets that regulate social behaviors. Our findings provide further evidence that adolescent enrichment can serve as an inoculating experience against future stressors. The ability to induce social resilience in a usually anxious line of animals by manipulating their environment has translational implications, as it underscores the feasibility of intervention strategies targeted at genetically vulnerable adolescent populations.

Early-life stress alters chromatin modifications in VTA to prime stress sensitivity

Early-life stress increases sensitivity to subsequent stress, which has been observed among humans, other animals, at the level of cellular activity, and at the level of gene expression. However, the molecular mechanisms underlying such long-lasting sensitivity are poorly understood. We tested the hypothesis that persistent changes in transcription and transcriptional potential were maintained at the level of the epigenome, through changes in chromatin. We used a combination of bottom-up mass spectrometry, viral-mediated epigenome-editing, behavioral quantification, and RNA-sequencing in a mouse model of early-life stress, focusing on the ventral tegmental area (VTA), a brain region critically implicated in motivation, reward learning, stress response, and mood and drug disorders. We find that early-life stress in mice alters histone dynamics in VTA and that a majority of these modifications are associated with an open chromatin state that would predict active, primed, or poised gene expression, including enriched histone-3 lysine-4 methylation and the H3K4 monomethylase Setd7. Mimicking ELS through over-expression of Setd7 and enrichment of H3K4me1 in VTA recapitulates ELS-induced behavioral and transcriptional hypersensitivity to future stress. These findings enrich our understanding of the epigenetic mechanisms linking early-life environmental experiences to long-term alterations in stress reactivity within the brain's reward circuitry, with implications for understanding and potentially treating mood and anxiety disorders in humans.

Thyroid hormones mediate the impact of early-life stress on ventral tegmental area gene expression and behavior

Proper thyroid function is essential to the developing brain, including dopamine neuron differentiation, growth, and maintenance. Stress across the lifespan impacts thyroid hormone signaling and anxiety disorders and depression have been associated with thyroid dysfunction (both hypo- and hyper-active). However, less is known about how stress during postnatal development impacts thyroid function and related brain development. Our previous work in mice demonstrated that early-life stress (ELS) transiently impinged on expression of a transcription factor in dopamine neurons, Otx2, shown to be regulated by thyroid hormones. We hypothesized that thyroid hormone signaling may link experience of ELS with transcriptional dysregulation within the dopaminergic midbrain, and ultimately behavior. Here, we find that ELS transiently increases thyroid-stimulating hormone levels (inversely related to thyroid signaling) in both male and female mice at P21, an effect which recovers by adolescence. We next tested whether transient treatment of ELS mice with synthetic thyroid hormone (levothyroxine, LT4) could ameliorate the impact of ELS on sensitivity to future stress, and on expression of genes related to dopamine neuron development and maintenance, thyroid signaling, and plasticity within the ventral tegmental area. Among male mice, but not females, juvenile LT4 treatment prevented hypersensitivity to adult stress. We also found that rescuing developmental deficits in thyroid hormone signaling after ELS restored levels of some genes altered directly by ELS, and prevented alterations in expression of other genes sensitive to the second hit of adult stress. These findings suggest that thyroid signaling mediates the deleterious impact of ELS on VTA development, and that temporary treatment of hypothyroidism after ELS may be sufficient to prevent future stress hypersensitivity.

Association of early life adversity with cardiovascular disease and its potential mechanisms: a narrative review

Strong epidemiological evidence has shown that early life adversity (ELA) has a profound negative impact on health in adulthood, including an increased risk of cardiovascular disease, the leading cause of death worldwide. Here, we review cohort studies on the effects of ELA on cardiovascular outcomes and the possible underlying mechanisms. In addition, we summarize relevant studies in rodent models of ELA. This review reveals that the prevalence of ELA varies between regions, time periods, and sexes. ELA increases cardiovascular health risk behaviors, susceptibility to mental illnesses, and neuroendocrine and immune system dysfunction in humans. Rodent models of ELA have been developed and show similar cardiovascular outcomes to those in humans but cannot fully replicate all ELA subtypes. Therefore, combining cohort and rodent studies to further investigate the mechanisms underlying the association between ELA and cardiovascular diseases may be a feasible future research strategy.

The impact of pubertal stress and adult hormone exposure on the transcriptome of the developing hypothalamus

Why individuals suffer negative consequences following stress is a complex phenomenon that is dictated by individual factors, the timing of stress within the lifespan, and when in the lifespan the consequences are measured. Women who undergo adverse childhood experiences are at risk for lasting biological consequences, including affective and stress dysregulation. We have shown that pubertal adversity is associated with a blunted hypothalamic-pituitary-adrenal axis glucocorticoid response in peripartum humans and mice. In mice, our prior examination of the paraventricular nucleus (PVN) of the hypothalamus showed that pubertal stress led to an upregulation of baseline mRNA expression of six immediate early genes (IEGs) in the PVN of adult, pregnant mice. Separately, we showed that the pregnancy-associated hormone allopregnanolone is necessary and sufficient to produce the blunted stress response phenotype in pubertally stressed mice. In the current study, we further examined a potential mechanistic role for the IEGs in the PVN. We found that in pubertally stressed adult female, but not male, mice, intra-PVN allopregnanolone was sufficient to recapitulate the baseline IEG mRNA expression profile previously observed in pubertally stressed, pregnant mice. We also examined baseline IEG mRNA expression during adolescence, where we found that IEGs have developmental trajectories that showed sex-specific disruption by pubertal stress. Altogether, these data establish that IEGs may act as a key molecular switch involved in increased vulnerability to negative outcomes in adult, pubertally stressed animals. How the factors that produce vulnerability combine throughout the lifespan is key to our understanding of the etiology of stress-related disorders.

Failure to mate enhances investment in behaviors that may promote mating reward and impairs the ability to cope with stressors via a subpopulation of Neuropeptide F receptor neurons

Living in dynamic environments such as the social domain, where interaction with others determines the reproductive success of individuals, requires the ability to recognize opportunities to obtain natural rewards and cope with challenges that are associated with achieving them. As such, actions that promote survival and reproduction are reinforced by the brain reward system, whereas coping with the challenges associated with obtaining these rewards is mediated by stress-response pathways, the activation of which can impair health and shorten lifespan. While much research has been devoted to understanding mechanisms underlying the way by which natural rewards are processed by the reward system, less attention has been given to the consequences of failure to obtain a desirable reward. As a model system to study the impact of failure to obtain a natural reward, we used the well-established courtship suppression paradigm in Drosophila melanogaster as means to induce repeated failures to obtain sexual reward in male flies. We discovered that beyond the known reduction in courtship actions caused by interaction with non-receptive females, repeated failures to mate induce a stress response characterized by persistent motivation to obtain the sexual reward, reduced male-male social interaction, and enhanced aggression. This frustrative-like state caused by the conflict between high motivation to obtain sexual reward and the inability to fulfill their mating drive impairs the capacity of rejected males to tolerate stressors such as starvation and oxidative stress. We further show that sensitivity to starvation and enhanced social arousal is mediated by the disinhibition of a small population of neurons that express receptors for the fly homologue of neuropeptide Y. Our findings demonstrate for the first time the existence of social stress in flies and offers a framework to study mechanisms underlying the crosstalk between reward, stress, and reproduction in a simple nervous system that is highly amenable to genetic manipulation.

The cortisol switch between vulnerability and resilience

In concert with neuropeptides and transmitters, the end products of the hypothalamus-pituitary-adrenal (HPA) axis, the glucocorticoid hormones cortisol and corticosterone (CORT), promote resilience: i.e., the ability to cope with threats, adversity, and trauma. To exert this protective action, CORT activates mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) that operate in a complementary manner -as an on/off switch- to coordinate circadian events, stress-coping, and adaptation. The evolutionary older limbic MR facilitates contextual memory retrieval and supports an on-switch in the selection of stress-coping styles at a low cost. The rise in circulating CORT concentration after stress subsequently activates a GR-mediated off-switch underlying recovery of homeostasis by providing the energy for restraining the primary stress reactions and promoting cognitive control over emotional reactivity. GR activation facilitates contextual memory storage of the experience to enable future stress-coping. Such complementary MR-GR-mediated actions involve rapid non-genomic and slower gene-mediated mechanisms; they are time-dependent, conditional, and sexually dimorphic, and depend on genetic background and prior experience. If coping fails, GR activation impairs cognitive control and promotes emotional arousal which eventually may compromise resilience. Such breakdown of resilience involves a transition to a chronic stress construct, where information processing is crashed; it leads to an imbalanced MR-GR switch and hence increased vulnerability. Novel MR-GR modulators are becoming available that may reset a dysregulated stress response system to reinstate the cognitive flexibility required for resilience.

A mouse model of the 3-hit effects of stress: Genotype controls the effects of life adversities in females

Helplessness is a dysfunctional coping response to stressors associated with different psychiatric conditions. The present study tested the hypothesis that early and adult adversities cumulate to produce helplessness depending on the genotype (3-hit hypothesis of psychopathology). To this aim, we evaluated whether Chronic Unpredictable Stress (CUS) differently affected coping and mesoaccumbens dopamine (DA) responses to stress challenge by adult mice of the C57BL/6J (B6) and DBA/2J (D2) inbred strains depending on early life experience (Repeated Cross Fostering, RCF). Three weeks of CUS increased the helplessness expressed in the Forced Swimming Test (FST) and the Tail Suspension Test by RCF-exposed female mice of the D2 strain. Moreover, female D2 mice with both RCF and CUS experiences showed inhibition of the stress-induced extracellular DA outflow in the Nucleus Accumbens, as measured by in vivo microdialysis, during and after FST. RCF-exposed B6 mice, instead, showed reduced helplessness and increased mesoaccumbens DA release. The present results support genotype-dependent additive effects of early experiences and adult adversities on behavioral and neural responses to stress by female mice. To our knowledge, this is the first report of a 3-hit effect in an animal model. Finally, the comparative analyses of behavioral and neural phenotypes expressed by B6 and D2 mice suggest some translationally relevant hypotheses of genetic risk factors for psychiatric disorders.

Transcriptional signatures of early-life stress and antidepressant treatment efficacy

Individuals with a history of early-life stress (ELS) tend to have an altered course of depression and lower treatment response rates. Research suggests that ELS alters brain development, but the molecular changes in the brain following ELS that may mediate altered antidepressant response have not been systematically studied. Sex and gender also impact the risk of depression and treatment response. Here, we leveraged existing RNA sequencing datasets from 1) blood samples from depressed female- and male-identifying patients treated with escitalopram or desvenlafaxine and assessed for treatment response or failure; 2) the nucleus accumbens (NAc) of female and male mice exposed to ELS and/or adult stress; and 3) the NAc of mice after adult stress, antidepressant treatment with imipramine or ketamine, and assessed for treatment response or failure. We find that transcriptomic signatures of adult stress after a history of ELS correspond with transcriptomic signatures of treatment nonresponse, across species and multiple classes of antidepressants. Transcriptomic correspondence with treatment outcome was stronger among females and weaker among males. We next pharmacologically tested these predictions in our mouse model of early-life and adult social defeat stress and treatment with either chronic escitalopram or acute ketamine. Among female mice, the strongest predictor of behavior was an interaction between ELS and ketamine treatment. Among males, however, early experience and treatment were poor predictors of behavior, mirroring our bioinformatic predictions. These studies provide neurobiological evidence for molecular adaptations in the brain related to sex and ELS that contribute to antidepressant treatment response.

Brief and long maternal separation in C57Bl6J mice: behavioral consequences for the dam and the offspring

Introduction

Animal models, especially rodents, have become instrumental to experimentally investigate the effects of an adverse post-natal environment on the developing brain. For this purpose, maternal separation (MS) paradigms have been widely used in the last decades. Nonetheless, how MS affects maternal behavior and, ultimately, the offspring depend on multiple variables.

Methods

To gain further insights into the consequences of MS, we decided to thoroughly measure and compare the effects of short (15 min, 3 times/day) vs. long (3 h, 1 time/day) separation on multiple maternally-associated behaviors and across the entire post-natal period.

Results

Compared to unhandled control litters, our results confirmed previous studies and indicated that SMS enhanced the time and variety of maternal care whereas LMS resulted in poor caregiving. We also showed that SMS-accrued caregiving persisted during the whole post-natal period. In contrast, LMS effects on maternal behavior were restricted to the early life (P2-P10). Finally, we also analyzed the behavioral consequences of these different rearing social environments on the offspring. We found that MS has profound effects in social tasks. We showed that affiliative touch, a type of prosocial behavior that provides comfort to others, is particularly sensitive to the modification of maternal caregiving.

Discussion

Our results provide further support to the contention that interactions during the early post-natal period critically contribute to emotional processing and brain co-construction.

Reconfiguration of brain-wide neural activity after early life adversity

Early life adversity (ELA) predisposes individuals to both physical and mental disorders lifelong. How ELA affects brain function leading to this vulnerability is under intense investigation. Research has begun to shed light on ELA effects on localized brain regions within defined circuits. However, investigations into brain-wide neural activity that includes multiple localized regions, determines relationships of activity between regions and identifies shifts of activity in response to experiential conditions is necessary. Here, we performed longitudinal manganese-enhanced magnetic resonance imaging (MEMRI) to image the brain in normally reared or ELA-exposed adults. Images were captured in the freely moving home cage condition, and short- and long-term after naturalistic threat. Images were analyzed with new computational methods, including automated segmentation and fractional activation or difference volumes. We found that neural activity was increased after ELA compared to normal rearing in multiple brain regions, some of which are involved in defensive and/or reward circuitry. Widely distributed patterns of neural activity, "brain states", and their dynamics after threat were altered with ELA. Upon acute threat, ELA-mice retained heightened neural activity within many of these regions, and new hyperactive responses emerged in monoaminergic centers of the mid- and hindbrain. Nine days after acute threat, heightened neural activity remained within locus coeruleus and increased within posterior amygdala, ventral hippocampus, and dorso- and ventromedial hypothalamus, while reduced activity emerged within medial prefrontal cortical regions (prelimbic, infralimbic, anterior cingulate). These results reveal that functional imbalances arise between multiple brain-systems which are dependent upon context and cumulative experiences after ELA.

Thyroid hormones mediate the impact of early-life stress on ventral tegmental area gene expression and behavior

Proper thyroid function is essential to the developing brain, including dopamine neuron differentiation, growth, and maintenance. Stress across the lifespan impacts thyroid hormone signaling and anxiety disorders and depression have been associated with thyroid dysfunction (both hypo- and hyper-active). However, less is known about how stress during postnatal development impacts thyroid function and related brain development. Our previous work in mice demonstrated that early-life stress (ELS) transiently impinged on expression of a transcription factor in dopamine neurons shown to be regulated by thyroid hormones. We hypothesized that thyroid hormone signaling may link experience of ELS with transcriptional dysregulation within the dopaminergic midbrain, and ultimately behavior. Here, we find that ELS transiently increases thyroid-stimulating hormone levels (inversely related to thyroid signaling) in both male and female mice at P21, an effect which recovers by adolescence. We next tested whether transient treatment of ELS mice with synthetic thyroid hormone (levothyroxine, LT4) could ameliorate the impact of ELS on sensitivity to future stress, and on expression of genes related to dopamine neuron development and maintenance, thyroid signaling, and plasticity within the ventral tegmental area. Among male mice, but not females, juvenile LT4 treatment prevented hypersensitivity to adult stress. We also found that rescuing developmental deficits in thyroid hormone signaling after ELS restored levels of some genes altered directly by ELS, and prevented alterations in expression of other genes sensitive to the second hit of adult stress. These findings suggest that thyroid signaling mediates the deleterious impact of ELS on VTA development, and that temporary treatment of hypothyroidism after ELS may be sufficient to prevent future stress hypersensitivity.

Global research trends on maternal separation paradigms as an early life stress model: A bibliometric analysis

Background

Maternal separation (MS) is an early life stress model that is often studied to determine how early life stress affects brain development and psychopathological adaptation. As society has developed, public health problems have become increasingly prominent, and this research area has attracted significant attention. However, to date, there has been no systematic bibliometric study on MS. The aim of this study was to analyze the trends and frontiers in MS using bibliometrics and provide a scientific reference to researchers in the field.

Methods

Utilizing VOSviewer, CiteSpace, and Microsoft Excel, examined data obtained from the WoSCC, which encompasses the years 2002-2021.

Results

In this bibliometric study, we analyzed 6209 articles related to MS authored by 24,174 researchers across 121 countries and regions and published in 2219 journals. The United States had the most publications (2,232, 35.95%) and both the United States and the United Kingdom had the highest h-index. Institutions in the United States and France had the most published articles and citations. Keyword clustering analysis revealed associations between MS and adverse early life experiences, the hypothalamic-pituitary-adrenal (HPA) axis, stress, gene expression, and depression.

Conclusions

This bibliometric analysis highlights the current research focus on the long-term effects of MS on emotional cognition, the HPA axis, epigenetic changes, and their links to gut microbiome imbalances. Future research may expand on these findings to investigate the underlying mechanisms and broader health and societal implications of MS. These results provide a comprehensive overview of the current research landscape in MS and offer valuable insights for researchers to guide future investigations in this field.

AKT and MAPK signaling pathways in hippocampus reveals the pathogenesis of depression in four stress-induced models

Major depressive disorder (MDD) is a highly heterogeneous psychiatric disorder. The pathogenesis of MDD remained unclear, and it may be associated with exposure to different stressors. Most previous studies have focused on molecular changes in a single stress-induced depression model, which limited the identification of the pathogenesis of MDD. The depressive-like behaviors were induced by four well-validated stress models in rats, including chronic unpredictable mild stress, learned helplessness stress, chronic restraint stress and social defeat stress. We applied proteomic and metabolomic to investigate molecular changes in the hippocampus of those four models and revealed 529 proteins and 98 metabolites. Ingenuity Pathways Analysis (IPA) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis identified differentially regulated canonical pathways, and then we presented a schematic model that simulates AKT and MAPK signaling pathways network and their interactions and revealed the cascade reactions. Further, the western blot confirmed that p-AKT, p-ERK12, GluA1, p-MEK1, p-MEK2, p-P38, Syn1, and TrkB, which were changed in at least one depression model. Importantly, p-AKT, p-ERK12, p-MEK1 and p-P38 were identified as common alterations in four depression models. The molecular level changes caused by different stressors may be dramatically different, and even opposite, between four depression models. However, the different molecular alterations converge on a common AKT and MAPK molecular pathway. Further studies of these pathways could contribute to a better understanding of the pathogenesis of depression, with the ultimate goal of helping to develop or select more effective treatment strategies for MDD.

The impact of environmental factors during maternal separation on the behaviors of adolescent C57BL/6 mice

Neonatal maternal separation is a widely used method to construct an early-life stress model in rodents. In this method, pups are separated from their mothers for several hours every day during the first 2 weeks of life, which results in adverse early-life events. It is a known fact that maternal separation can exert a significant impact on the behavior and psychological health, such as anxiety and depression, in adolescent offspring. However, environmental conditions during maternal separation can differ such as the presence of other animals or by placing pups in a different dam. To investigate the differential effects of various conditions of maternal separation on the behavior of adolescent mice, we created the following groups: (1) iMS group: pups were moved to an isolated room with no other adult mice in a nearby cage, (2) eDam group: the pups randomly exchanged their dams, (3) OF group: pups were shifted to another cage with the bedding material containing maternal odor (olfactory stimulation), and (4) MS group: pups were shifted to another vivarium. From postnatal day (PND) 2-20 (i.e., 19 consecutive days), pups were separated from the dam daily for 4 h and exposed to various environments (MS, iMS, eDam, and OF) or were left undisturbed [control (CON) group]. A series of behavioral assessments were conducted to evaluate locomotion, anxiety, recognition, learning, and memory in adolescent offspring. The results showed that neonatal maternal separation led to impaired recognition memory, motor coordination, and motor skill learning across all groups. However, the iMS group exhibited anxiety-like behavior in the elevated plus maze test and enhanced the extinction of fear memory in the auditory fear conditioning test. The OF and eDam groups displayed partially recovered short-term working memory in the Y-maze test but exhibited opposite exploratory behaviors. The OF group spent more time in the center, while the eDam group spent less time. These findings demonstrated that exposure to different environmental conditions during maternal separation causes behavioral alterations in adolescent offspring, providing a potential explanation for the variation in behavioral phenotypes observed in the early-life stress models.

Chronic exposure to aflatoxin B1 increases hippocampal microglial pyroptosis and vulnerability to stress in mice

BACKGROUND

Chronic aflatoxin B1 (AFB1) exposure may increase the risk of multiple neuropsychiatric disorders. Stress is considered one of the main contributors to major depressive disorder. Whether and how chronic AFB1 exposure affects vulnerability to stress is unclear.

METHODS

Mice were exposed for three weeks to AFB1 (100 µg/kg/d) and/or chronic mild stress (CMS). The vulnerability behaviors in response to stress were assessed in the forced swimming test (FST), sucrose preference test (SPT), and tail suspension test (TST). Microglial pyroptosis was investigated using immunofluorescence, enzyme-linked immunosorbent assays, and western blot assay in the hippocampus of mice. Hippocampal neurogenesis and the effects of AFB1-treated microglia on proliferation and differentiation of neural stem/precursor cells (NSPCs) were assessed via immunofluorescence in the hippocampus of mice.

RESULTS

Mice exposed to CMS in the presence of AFB1 exhibited markedly greater vulnerability to stress than mice treated with CMS or AFB1 alone, as indicated by reduced sucrose preference and longer immobility time in the forced swimming test. Chronic aflatoxin B1 exposure resulted in changes in the microglial morphology and increase in TUNEL⁺ microglia and GSDMD⁺ microglia in the hippocampal dentate gyrus. When mice were exposed to both CMS and AFB1, pyroptosis-related molecules (such as NLRP3, caspase-1, GSDMD-N, and interleukin-1β) were significantly upregulated in the hippocampus. These molecules were also significantly enhanced by AFB1 in primary microglial cultures. AFB1-treated mice showed decrease in the numbers of BrdU⁺, BrdU-DCX⁺, and BrdU-NeuN⁺ cells in the hippocampal dentate gyrus, as well as the percentages of BrdU⁺ cells that were NeuN⁺ in the presence or absence of CMS when compared with vehicle-treated mice. The combination of AFB1 and CMS exacerbated these effects to an even greater extent. The number of DCX⁺ cells correlated negatively with the percentage of ameboid microglia, TUNEL⁺ microglia and GSDMD⁺ microglia in the hippocampal dentate gyrus. AFB1-treated microglia suppressed the proliferation and neuronal differentiation of NSPCs in vitro.

CONCLUSION

Chronic AFB1 exposure induces microglial pyroptosis, promoting an adverse neurogenic microenvironment that impairs hippocampal neurogenesis, which may render mice more vulnerable to stress.

Intergenerational response to sperm competition risk in an invasive mammal

Studies of socially mediated phenotypic plasticity have demonstrated adaptive male responses to the 'competitive' environment. Despite this, whether variation in the paternal social environment also influences offspring reproductive potential in an intergenerational context has not yet been examined. Here, we studied the descendants of wild-caught house mice, a destructive pest species worldwide, to address this knowledge gap. We analysed traits that define a 'competitive' phenotype in the sons of males (sires) that had been exposed to either a high-male density (competitive) or high-female density (non-competitive) environment. We report disparate reproductive strategies among the sires: high-male density led to a phenotype geared for competition, while high-female density led to a phenotype that would facilitate elevated mating frequency. Moreover, we found that the competitive responses of sires persisted in the subsequent generation, with the sons of males reared under competition having elevated sperm quality. As all sons were reared under common-garden conditions, variation in their reproductive phenotypes could only have arisen via nongenetic inheritance. We discuss our results in relation to the adaptive advantage of preparing sons for sperm competition and suggest that intergenerational plasticity is a previously unconsidered aspect in invasive mammal fertility control.

Stress in adolescence as a first hit in stress-related disease development: Timing and context are crucial

The two-hit stress model predicts that exposure to stress at two different time-points in life may increase or decrease the risk of developing stress-related disorders later in life. Most studies based on the two-hit stress model have investigated early postnatal stress as the first hit with adult stress as the second hit. Adolescence, however, represents another highly sensitive developmental window during which exposure to stressful events may affect programming outcomes following exposure to stress in adulthood. Here, we discuss the programming effects of different types of stressors (social and nonsocial) occurring during adolescence (first hit) and how such stressors affect the responsiveness toward an additional stressor occurring during adulthood (second hit) in rodents. We then provide a comprehensive overview of the potential mechanisms underlying interindividual and sex differences in the resilience/susceptibility to developing stress-related disorders later in life when stress is experienced in two different life stages.

Dysregulation of Prefrontal Oligodendrocyte Lineage Cells Across Mouse Models of Adversity and Human Major Depressive Disorder Oligodendrocyte dysregulation in mouse models of stress and MDD

Animal models of adversity have yielded few molecular mechanisms that translate to human stress-related diseases like major depressive disorder (MDD). We congruently analyze publicly available bulk-tissue transcriptomic data from prefrontal cortex (PFC) in multiple mouse models of adversity and in MDD. We apply strategies, to quantify cell-type specific enrichment from bulk-tissue transcriptomics, utilizing reference single cell RNA sequencing datasets. These analyses reveal conserved patterns of oligodendrocyte (OL) dysregulation across animal experiments, including susceptibility to social defeat, acute cocaine withdrawal, chronic unpredictable stress, early life stress, and adolescent social isolation. Using unbiased methodologies, we further identify a dysregulation of layer 6 neurons that associate with deficits in goal-directed behavior after social isolation. Human post-mortem brains with MDD show similar OL transcriptome changes in Brodmann Areas 8/9 in both male and female patients. This work assesses cell type involvement in an unbiased manner from differential expression analyses across animal models of adversity and human MDD and finds a common signature of OL dysfunction in the frontal cortex.

Prophylactic Efficacy of Riluzole against Anxiety- and Depressive-Like Behaviors in Two Rodent Stress Models

Introduction

Chronic stress-related illnesses such as major depressive disorder and post-traumatic stress disorder share symptomatology, including anxiety, anhedonia, and helplessness. Across disorders, neurotoxic dysregulated glutamate (Glu) signaling may underlie symptom emergence. Current first-line antidepressant drugs, which do not directly target Glu signaling, fail to provide adequate benefit for many patients and are associated with high relapse rates. Riluzole modulates glutamatergic neurotransmission by increasing metabolic cycling and modulating signal transduction. Clinical studies exploring riluzole's efficacy in stress-related disorders have provided varied results. However, the utility of riluzole for treating specific symptom dimensions or as a prophylactic treatment has not been comprehensively assessed.

Methods

We investigated whether chronic prophylactic riluzole (∼12-15 mg/kg/day p.o.) could prevent the emergence of behavioral deficits induced by unpredictable chronic mild stress (UCMS) in mice. We assessed (i) anxiety-like behavior using the elevated-plus maze, open-field test, and novelty-suppressed feeding, (ii) mixed anxiety/anhedonia-like behavior in the novelty-induced hypophagia test, and (iii) anhedonia-like behavior using the sucrose consumption test. Z-scoring summarized changes across tests measuring similar dimensions. In a separate learned helplessness (LH) cohort, we investigated whether chronic prophylactic riluzole treatment could block the development of helplessness-like behavior.

Results

UCMS induced an elevation in anhedonia-like behavior and overall behavioral emotionality that was blocked by prophylactic riluzole. In the LH cohort, prophylactic riluzole blocked the development of helplessness-like behavior.

Discussion/Conclusion

This study supports the utility of riluzole as a prophylactic medication for preventing anhedonia and helplessness symptoms associated with stress-related disorders.

Stress while lacking of control induces ventral hippocampal autophagic flux hyperactivity and a depression-like behavior

BACKGROUND

Stressed animals may perform depression-like behavior insomuch as stress-provoking blood-brain barrier (BBB) disruption, central immune activation, and autophagic flux changes. This study was undertaken to assess whether adult mice having (executive) vs. lacking (yoke) of behavioral control in otherwise equivalent stress magnitude condition, may display differences in their BBB integrity, ventral hippocampal (VH) interleukin-6 (IL-6) and autophagic flux level and VH-related depression-like behavior. To further understand the causative relation of enhanced autophagic flux and stress-primed depression-like behavior, we assessed the effects of bilateral intra-VH 3-methyladenine (3-MA), an autophagic flux inhibitor, infusion in stressed mice.

METHODS

Adult mice used had comparable genetic background and housing condition. Executive/yoke pairs of mice received a 10-day (1 h/day) footshock stressor regimen. Throughout the regimen, the ongoing footshock was terminated immediately contingent on the executive mouse', while irrelevant to the respective yoke mouse' voluntary behavior, or lasting for 7 s. Each dyad's cage-mate receiving no such regimen served as no stressor controls.

RESULTS

Yoke mice displayed disrupted BBB integrity (escalated Evans blue extravasation and decreased VH ZO-1, claudin-5 expression), increases in VH autophagic flux (increased LC3II/LC3I and decreased p62) and immobility duration in forced swimming test. Most of these indices remained unaltered in executive mice. Administration of 3-MA did not affect immobility duration in control mice, while prevented the increases in immobility duration in yoke mice.

CONCLUSIONS

(1) stress susceptibility may be determined by their differences in stress-coping results; (2) VH autophagic flux increase plays a permissive role in priming the stressed animals susceptible to exhibit depression-like behavior.

Enduring effects of early-life adversity on reward processes: A systematic review and meta-analysis of animal studies

Two-thirds of individuals experience adversity during childhood such as neglect, abuse or highly-stressful events. Early-life adversity (ELA) increases the life-long risk of developing mood and substance use disorders. Reward-related deficits has emerged as a key endophenotype of such psychiatric disorders. Animal models are invaluable for studying how ELA leads to reward deficits. However, the existing literature is heterogenous with difficult to reconcile findings. To create an overview, we conducted a systematic review containing multiple meta-analyses regarding the effects of ELA on reward processes overall and on specific aspects of reward processing in animal models. A comprehensive search identified 120 studies. Most studies omitted key details resulting in unclear risk of bias. Overall meta-analysis showed that ELA significantly reduced reward behaviors (SMD: -0.42 [-0.60; -0.24]). The magnitude of ELA effects significantly increased with longer exposure. When reward domains were analyzed separately, ELA only significantly dampened reward responsiveness (SMD: -0.525[-0.786; -0.264]) and social reward processing (SMD: -0.374 [-0.663; -0.084]), suggesting that ELA might lead to deficits in specific reward domains.

The rearing environment persistently modulates mouse phenotypes from the molecular to the behavioural level

The phenotype of an organism results from its genotype and the influence of the environment throughout development. Even when using animals of the same genotype, independent studies may test animals of different phenotypes, resulting in poor replicability due to genotype-by-environment interactions. Thus, genetically defined strains of mice may respond differently to experimental treatments depending on their rearing environment. However, the extent of such phenotypic plasticity and its implications for the replicability of research findings have remained unknown. Here, we examined the extent to which common environmental differences between animal facilities modulate the phenotype of genetically homogeneous (inbred) mice. We conducted a comprehensive multicentre study, whereby inbred C57BL/6J mice from a single breeding cohort were allocated to and reared in 5 different animal facilities throughout early life and adolescence, before being transported to a single test laboratory. We found persistent effects of the rearing facility on the composition and heterogeneity of the gut microbial community. These effects were paralleled by persistent differences in body weight and in the behavioural phenotype of the mice. Furthermore, we show that environmental variation among animal facilities is strong enough to influence epigenetic patterns in neurons at the level of chromatin organisation. We detected changes in chromatin organisation in the regulatory regions of genes involved in nucleosome assembly, neuronal differentiation, synaptic plasticity, and regulation of behaviour. Our findings demonstrate that common environmental differences between animal facilities may produce facility-specific phenotypes, from the molecular to the behavioural level. Furthermore, they highlight an important limitation of inferences from single-laboratory studies and thus argue that study designs should take environmental background into account to increase the robustness and replicability of findings.

The phenotype of an organism results not only from its genotype but also the influence of its environment throughout development. This study shows that common environmental differences between animal facilities can induce substantial variation in the phenotype of mice, thereby highlighting an important limitation of inferences from single-laboratory studies in animal research.

Resource scarcity but not maternal separation provokes unpredictable maternal care sequences in mice and both upregulate Crh-associated gene expression in the amygdala

Early life adversity (ELA) is a major risk factor for the development of pathology, including anxiety disorders. Neurodevelopmental and behavioral outcomes following ELA are multifaceted and are influenced heavily by the type of adversity experienced and sex of the individual experiencing ELA. It remains unclear what properties of ELA portend differential neurobiological risk and the basis of sex-differences for negative outcomes. Predictability of the postnatal environment has emerged as being a core feature supporting development, with the most salient signals deriving from parental care. Predictability of parental care may be a distinguishing feature of different forms of ELA, and the degree of predictability afforded by these manipulations may contribute to the diversity of outcomes observed across models. Further, questions remain as to whether differing levels of predictability may contribute to differential effects on neurodevelopment and expression of genes associated with risk for pathology. Here, we tested the hypothesis that changes in maternal behavior in mice would be contingent on the type of ELA experienced, directly comparing predictability of care in the limited bedding and nesting (LBN) and maternal separation (MS) paradigms. We then tested whether the predictability of the ELA environment altered the expression of corticotropin-releasing hormone (Crh), a sexually-dimorphic neuropeptide that regulates threat-related learning, in the amygdala of male and female mice. The LBN manipulation reliably increased the entropy of maternal care, a measure that indicates lower predictability between sequences of dam behavior. LBN and MS rearing similarly increased the frequency of nest sorties and licking of pups but had mixed effects on other aspects of dam-, pup-, and nest-related behaviors. Increased expression of Crh-related genes was observed in pups that experienced ELA, with gene expression measures showing a significant interaction with sex and type of ELA manipulation. Specifically, MS was associated with increased expression of Crh-related genes in males, but not females, and LBN primarily increased expression of these genes in females, but not males. The present study provides evidence for predictability as a distinguishing feature of models of ELA and demonstrates robust consequences of these differing experience on sex-differences in gene expression critically associated with stress responding and sex differences in risk for pathology.

Using animal models for the studies of schizophrenia and depression: The value of translational models for treatment and prevention

Animal models of psychiatric disorders have been highly effective in advancing the field, identifying circuits related to pathophysiology, and identifying novel therapeutic targets. In this review, we show how animal models, particularly those based on development, have provided essential information regarding circuits involved in disorders, disease progression, and novel targets for intervention and potentially prevention. Nonetheless, in recent years there has been a pushback, largely driven by the US National Institute of Mental Health (NIMH), to shift away from animal models and instead focus on circuits in normal subjects. This has been driven primarily from a lack of discovery of new effective therapeutic targets, and the failure of targets based on preclinical research to show efficacy. We discuss why animal models of complex disorders, when strongly cross-validated by clinical research, are essential to understand disease etiology as well as pathophysiology, and direct new drug discovery. Issues related to shortcomings in clinical trial design that confound translation from animal models as well as the failure to take patient pharmacological history into account are proposed to be a source of the failure of what are likely effective compounds from showing promise in clinical trials.

Early Life Stress-Induced Epigenetic Programming of Hippocampal NPY-Y2 Receptor Gene Expression Changes in Response to Adult Stress

Early Life Stress (ELS) can critically influence brain development and future stress responses and thus represents an important risk factor for mental health and disease. Neuropeptide Y (NPY) is discussed to be a key mediator of resilient vs. vulnerable adaptations and specifically, the NPY-Y2 receptor (Y2R) may be involved in the pathophysiology of depression due to its negative regulation of NPY-release. The present study addressed the hypotheses that ELS and adult stress (AS) affect the expression of hippocampal Y2R and that exposure to ELS induces an epigenetically mediated programming effect towards a consecutive stress exposure in adulthood. The specific aims were to investigate if (i) ELS or AS as single stressors induce changes in Y2 receptor gene expression in the hippocampus, (ii) the predicted Y2R changes are epigenetically mediated via promoter-specific DNA-methylation, (iii) the ELS-induced epigenetic changes exert a programming effect on Y2R gene expression changes in response to AS, and finally (iv) if the predicted alterations are sex-specific. Animals were assigned to the following experimental groups: (1) non-stressed controls (CON), (2) only ELS exposure (ELS), (3) only adult stress exposure (CON+AS), and (4) exposure to ELS followed by AS (ELS+AS). Using repeated maternal separation in mice as an ELS and swim stress as an AS we found that both stressors affected Y2R gene expression in the hippocampus of male mice but not in females. Specifically, upregulated expression was found in the CON+AS group. In addition, exposure to both stressors ELS+AS significantly reduced Y2R gene expression when compared to CON+AS. The changes in Y2R expression were paralleled by altered DNA-methylation patterns at the Y2R promoter, specifically, a decrease in mean DNA-methylation in the CON+AS males compared to the non-AS exposed groups and an increase in the ELS+AS males compared to the CON+AS males. Also, a strong negative correlation of mean DNA-methylation with Y2R expression was found. Detailed CpG-site-specific analysis of DNA-methylation revealed that ELS induced increased DNA-methylation only at specific CpG-sites within the Y2R promoter. It is tempting to speculate that these ELS-induced CpG-site-specific changes represent a “buffering” programming effect against elevations of Y2R expression induced by AS.

Early Life Adversity and Neuropsychiatric Disease: Differential Outcomes and Translational Relevance of Rodent Models

It is now well-established that early life adversity (ELA) predisposes individuals to develop several neuropsychiatric conditions, including anxiety disorders, and major depressive disorder. However, ELA is a very broad term, encompassing multiple types of negative childhood experiences, including physical, sexual and emotional abuse, physical and emotional neglect, as well as trauma associated with chronic illness, family separation, natural disasters, accidents, and witnessing a violent crime. Emerging literature suggests that in humans, different types of adverse experiences are more or less likely to produce susceptibilities to certain conditions that involve affective dysfunction. To investigate the driving mechanisms underlying the connection between experience and subsequent disease, neuroscientists have developed several rodent models of ELA, including pain exposure, maternal deprivation, and limited resources. These studies have also shown that different types of ELA paradigms produce different but somewhat overlapping behavioral phenotypes. In this review, we first investigate the types of ELA that may be driving different neuropsychiatric outcomes and brain changes in humans. We next evaluate whether rodent models of ELA can provide translationally relevant information regarding links between specific types of experience and changes in neural circuits underlying dysfunction.

Neurodevelopmental origins of substance use disorders: Evidence from animal models of early-life adversity and addiction

Addiction is a chronic relapsing disorder with devastating personal, societal, and economic consequences. In humans, early-life adversity (ELA) such as trauma, neglect, and resource scarcity are linked with increased risk of later-life addiction, but the brain mechanisms underlying this link are still poorly understood. Here, we focus on data from rodent models of ELA and addiction, in which causal effects of ELA on later-life responses to drugs and the neurodevelopmental mechanisms by which ELA increases vulnerability to addiction can be determined. We first summarize evidence for a link between ELA and addiction in humans, then describe how ELA is commonly modeled in rodents. Since addiction is a heterogeneous disease with many individually varying behavioral aspects that may be impacted by ELA, we next discuss common rodent assays of addiction-like behaviors. We then summarize the specific addiction-relevant behavioral phenotypes caused by ELA in male and female rodents and discuss some of the underlying changes in brain reward and stress circuits that are likely responsible. By better understanding the behavioral and neural mechanisms by which ELA promotes addiction vulnerability, we hope to facilitate development of new approaches for preventing or treating addiction in those with a history of ELA.

Short-Chain Fatty Acids Ameliorate Depressive-Like Behaviors of High Fructose-Fed Mice by Rescuing Hippocampal Neurogenesis Decline and Blood–Brain Barrier Damage

Excessive fructose intake is associated with the increased risk of mental illness, such as depression, but the underlying mechanisms are poorly understood. Our previous study found that high fructose diet (FruD)-fed mice exhibited neuroinflammation, hippocampal neurogenesis decline and blood–brain barrier (BBB) damage, accompanied by the reduction of gut microbiome-derived short-chain fatty acids (SCFAs). Here, we found that chronic stress aggravated these pathological changes and promoted the development of depressive-like behaviors in FruD mice. In detail, the decreased number of newborn neurons, mature neurons and neural stem cells (NSCs) in the hippocampus of FruD mice was worsened by chronic stress. Furthermore, chronic stress exacerbated the damage of BBB integrity with the decreased expression of zonula occludens-1 (ZO-1), claudin-5 and occludin in brain vasculature, overactivated microglia and increased neuroinflammation in FruD mice. These results suggest that high fructose intake combined with chronic stress leads to cumulative negative effects that promote the development of depressive-like behaviors in mice. Of note, SCFAs could rescue hippocampal neurogenesis decline, improve BBB damage and suppress microglia activation and neuroinflammation, thereby ameliorate depressive-like behaviors of FruD mice exposed to chronic stress. These results could be used to develop dietary interventions to prevent depression.

Genomic modules and intramodular network concordance in susceptible and resilient male mice across models of stress

The multifactorial etiology of stress-related disorders necessitates a constant interrogation of the molecular convergences in preclinical models of stress that use disparate paradigms as stressors spanning from environmental challenges to genetic predisposition to hormonal signaling. Using RNA-sequencing, we investigated the genomic signatures in the ventral hippocampus common to mouse models of stress. Chronic oral corticosterone (CORT) induced increased anxiety- and depression-like behavior in wild-type male mice and male mice heterozygous for the gene coding for brain-derived neurotrophic factor Val66Met, a variant associated with genetic susceptibility to stress. In a separate set of male mice, chronic social defeat stress (CSDS) led to a susceptible or a resilient population, whose proportion was dependent on housing conditions, namely standard housing or enriched environment. Rank-rank-hypergeometric overlap (RRHO), a threshold-free approach that ranks genes by their p value and effect size direction, was used to identify genes from a continuous gradient of significancy that were concordant across groups. In mice treated with CORT and in standard-housed susceptible mice, differentially expressed genes (DEGs) were concordant for gene networks involved in neurotransmission, cytoskeleton function, and vascularization. Weighted gene co-expression analysis generated 54 gene hub modules and revealed two modules in which both CORT and CSDS-induced enrichment in DEGs, whose function was concordant with the RRHO predictions, and correlated with behavioral resilience or susceptibility. These data showed transcriptional concordance across models in which the stress coping depends upon hormonal, environmental, or genetic factors revealing common genomic drivers that embody the multifaceted nature of stress-related disorders.

Genome-wide Signatures of Early-Life Stress: Influence of Sex

Both history of early-life stress (ELS) and female sex are associated with increased risk for depression. The complexity of how ELS interacts with brain development and sex to impart risk for multifaceted neuropsychiatric disorders is also unlikely to be understood by examining changes in single genes. Here, we review an emerging literature on genome-wide transcriptional and epigenetic signatures of ELS and the potential moderating influence of sex. We discuss evidence both that there are latent sex differences revealed by ELS and that ELS itself produces latent transcriptomic changes revealed by adult stress. In instances where there are broad similarities in global signatures of ELS among females and males, genes that contribute to these patterns are largely distinct based on sex. As this area of investigation grows, an effort should be made to better understand the sex-specific impact of ELS within the human brain, specific contributions of chromosomal versus hormonal sex, how ELS alters the time course of normal transcriptional development, and the cell-type specificity of transcriptomic and epigenomic changes in the brain. A better understanding of how ELS interacts with sex to alter transcriptomic and epigenomic signatures in the brain will inform individualized therapeutic strategies to prevent or ameliorate depression and other psychiatric disorders in this vulnerable population.

Family environment and development in children adopted from institutionalized care

BACKGROUND

After adoption, children exposed to institutionalized care show significant improvement, but incomplete recovery of growth and developmental milestones. There is a paucity of data regarding risk and protective factors in children adopted from institutionalized care. This prospective study followed children recently adopted from institutionalized care to investigate the relationship between family environment, executive function, and behavioral outcomes.

METHODS

Anthropometric measurements, physical examination, endocrine and bone age evaluations, neurocognitive testing, and behavioral questionnaires were evaluated over a 2-year period with children adopted from institutionalized care and non-adopted controls.

RESULTS

Adopted children had significant deficits in growth, cognitive, and developmental measurements compared to controls that improved; however, residual deficits remained. Family cohesiveness and expressiveness were protective influences, associated with less behavioral problems, while family conflict and greater emphasis on rules were associated with greater risk for executive dysfunction.

CONCLUSIONS

Our data suggest that a cohesive and expressive family environment moderated the effect of pre-adoption adversity on cognitive and behavioral development in toddlers, while family conflict and greater emphasis on rules were associated with greater risk for executive dysfunction. Early assessment of child temperament and parenting context may serve to optimize the fit between parenting style, family environment, and the child's development.

IMPACT

Children who experience institutionalized care are at increased risk for significant deficits in developmental, cognitive, and social functioning associated with a disruption in the development of the prefrontal cortex. Aspects of the family caregiving environment moderate the effect of early life social deprivation in children. Family cohesiveness and expressiveness were protective influences, while family conflict and greater emphasis on rules were associated with a greater risk for executive dysfunction problems. This study should be viewed as preliminary data to be referenced by larger studies investigating developmental and behavioral outcomes of children adopted from institutional care.

Early Life Painful Procedures: Long-Term Consequences and Implications for Farm Animal Welfare

Farm animals routinely undergo painful husbandry procedures early in life, including disbudding and castration in calves and goat kids, tail docking and castration in piglets and lambs, and beak trimming in chicks. In rodents, inflammatory events soon after birth, when physiological systems are developing and sensitive to perturbation, can profoundly alter phenotypic outcomes later in life. This review summarizes the current state of research on long-term phenotypic consequences of neonatal painful procedures in rodents and farm animals, and discusses the implications for farm animal welfare. Rodents exposed to early life inflammation show a hypo-/hyper-responsive profile to pain-, fear-, and anxiety-inducing stimuli, manifesting as an initial attenuation in responses that transitions into hyperresponsivity with increasing age or cumulative stress. Neonatal inflammation also predisposes rodents to cognitive, social, and reproductive deficits, and there is some evidence that adverse effects may be passed to offspring. The outcomes of neonatal inflammation are modulated by injury etiology, age at the time of injury and time of testing, sex, pain management, and rearing environment. Equivalent research examining long-term phenotypic consequences of early life painful procedures in farm animals is greatly lacking, despite obvious implications for welfare and performance. Improved understanding of how these procedures shape phenotypes will inform efforts to mitigate negative outcomes through reduction, replacement, and refinement of current practices.

Early Adversity and Development: Parsing Heterogeneity and Identifying Pathways of Risk and Resilience

Adversity early in life is common and is a major risk factor for the onset of psychopathology. Delineating the neurodevelopmental pathways by which early adversity affects mental health is critical for early risk identification and targeted treatment approaches. A rapidly growing cross-species literature has facilitated advances in identifying the mechanisms linking adversity with psychopathology, specific dimensions of adversity and timing-related factors that differentially relate to outcomes, and protective factors that buffer against the effects of adversity. Yet, vast complexity and heterogeneity in early environments and neurodevelopmental trajectories contribute to the challenges of understanding risk and resilience in the context of early adversity. In this overview, the author highlights progress in four major areas-mechanisms, heterogeneity, developmental timing, and protective factors; synthesizes key challenges; and provides recommendations for future research that can facilitate progress in the field. Translation across species and ongoing refinement of conceptual models have strong potential to inform prevention and intervention strategies that can reduce the immense burden of psychopathology associated with early adversity.

Impact of Early Life Stress on Reward Circuit Function and Regulation

Early life stress - including experience of child maltreatment, neglect, separation from or loss of a parent, and other forms of adversity - increases lifetime risk of mood, anxiety, and substance use disorders. A major component of this risk may be early life stress-induced alterations in motivation and reward processing, mediated by changes in the nucleus accumbens (NAc) and ventral tegmental area (VTA). Here, we review evidence of the impact of early life stress on reward circuit structure and function from human and animal models, with a focus on the NAc. We then connect these results to emerging theoretical models about the indirect and direct impacts of early life stress on reward circuit development. Through this review and synthesis, we aim to highlight open research questions and suggest avenues of future study in service of basic science, as well as applied insights. Understanding how early life stress alters reward circuit development, function, and motivated behaviors is a critical first step toward developing the ability to predict, prevent, and treat stress-related psychopathology spanning mood, anxiety, and substance use disorders.

Early life adoption shows rearing environment supersedes transgenerational effects of paternal stress on aggressive temperament in the offspring

Prenatal experience and transgenerational influences are increasingly recognized as critical for defining the socio-emotional system, through the development of social competences and of their underlying neural circuitries. Here, we used an established rat model of social stress resulting from male partner aggression induced by peripubertal (P28-42) exposure to unpredictable fearful experiences. Using this model, we aimed to first, characterize adult emotionality in terms of the breadth of the socio-emotional symptoms and second, to determine the relative impact of prenatal vs postnatal influences. For this purpose, male offspring of pairs comprising a control or a peripubertally stressed male were cross-fostered at birth and tested at adulthood on a series of socio-emotional tests. In the offspring of peripubertally stressed males, the expected antisocial phenotype was observed, as manifested by increased aggression towards a female partner and a threatening intruder, accompanied by lower sociability. This negative outcome was yet accompanied by better social memory as well as enhanced active coping, based on more swimming and longer latency to immobility in the forced swim test, and less immobility in the shock probe test. Furthermore, the cross-fostering manipulation revealed that these adult behaviors were largely influenced by the post- but not the prenatal environment, an observation contrasting with both pre- and postnatal effects on attacks during juvenile play behavior. Adult aggression, other active coping behaviors, and social memory were determined by the predominance at this developmental stage of postnatal over prenatal influences. Together, our data highlight the relative persistence of early life influences.

MicroRNA Regulates Early-Life Stress–Induced Depressive Behavior via Serotonin Signaling in a Sex-Dependent Manner in the Prefrontal Cortex of Rats

Background

The underlying neurobiology of early-life stress (ELS)-induced major depressive disorder is not clearly understood.

Methods

In this study, we used maternal separation (MS) as a rodent model of ELS and tested whether microRNAs (miRNAs) target serotonin genes to regulate ELS-induced depression-like behavior and whether this effect is sex dependent. We also examined whether environmental enrichment prevents susceptibility to depression- and anxiety-like behavior following MS and whether enrichment effects are mediated through serotonin genes and their corresponding miRNAs.

Results

MS decreased sucrose preference, which was reversed by enrichment. Males also exhibited greater changes in forced swim climbing and escape latency tests only following enrichment. Slc6a4 and Htr1a were upregulated in the frontal cortex following MS. In male MS rats, enrichment slightly reversed Htr1a expression to levels similar to control rats. miR-200a-3p and miR-322-5p, which target SLC6A4, were decreased by MS, but not significantly. An HTR1A-targeting miRNA, miR-320-5p, was also downregulated by MS and showed slight reversal by enrichment in male animals. miR-320-5p targeting of Htr1a was validated in vitro using SHSY neuroblastoma cell lines.

Conclusions

Altogether, this study implicates miRNA interaction with the serotonin pathway in ELS-induced susceptibility to depression-related reward deficits. Furthermore, because of its recovery by enrichment in males, miR-320 may represent a viable sex-specific target for reward-related deficits in major depressive disorder.

Adolescent oxytocin administration reduces depression-like behaviour induced by early life stress in adult male and female rats

Early life stress (ELS) exposure alters brain development, increasing vulnerability for mental illness in adulthood, including depression. Despite this association, there are no approved pharmacotherapies to protect against the emergence of mental illness resulting from ELS. Recent preclinical work showed that oxytocin (OT) administration in adulthood reduced depressive-like behaviour in male rats with a history of ELS. However, the ability of an OT treatment regime in adolescence, a critical developmental window for the OT system, to prevent the expression of depressive-like behaviours following ELS has not been investigated. Therefore, the present study aimed to determine whether chronic OT administration can ameliorate the enduring effects of ELS on depressive-like behaviours in both male and female rats. Following birth, Long Evans rat pups (N = 107) underwent maternal separation (MS) for either 15 min (MS15) or 6 h (MS360) on postnatal days (PND) 1-21. During adolescence (PND 28-42), rats received a daily injection of either OT (1 mg/kg) or saline. During adulthood (PND 57 onwards), effort-related motivation was measured using a model of effortful choice (EC), while behavioural despair was measured using the forced swim test (FST). Lastly, body and organ weights were measured to examine the physiological impacts of ELS and chronic OT administration. Overall, in both sexes, MS360 increased behavioural despair yet had no impact on effort-related motivation. Importantly, adolescent OT administration prevented the MS360-induced increase in behavioural despair in both males and females. Additionally, MS360 resulted in persistent reductions in body weight in both sexes post-weaning and increased spleen weight in males and adrenal weight in females. OT treatment had no impact on body weight in either sex, but prevented the MS-induced increase in adrenal gland weight in females. Overall, these findings have important implications for using oxytocin as a preventative pharmacotherapy after ELS.